Microbially-stable potato products and process

ABSTRACT

Gelatinized potato pieces containing sufficient generally uniformly dispersed sulfur dioxide to enable the pieces to be protected from microbial contamination even when stored for prolonged periods at ambient temperature. An article of manufacture including ambient temperature, stable, sulfur dioxide-containing gelatinized potato pieces surrounded by a substantially oxygen free environment in a microbe/oxygen impermeable container. The process of treating gelatinized potato pieces to make them microbially stable at ambient temperature during prolonged time periods by dispersing sulfur dioxide through the pieces and protecting them from subsequent contamination. Storage of the pieces in a substantially oxygen free or inert gas environment also enhances flavor stability in the stored products. Products include partially cooked french fry strips which need only to be heated or fried to make ready for consumption, as well as large pieces such as whole potatoes which can be used in such food products as potato salad with no further treatment or heated to make a baked potato.

This is a division of application Ser. No. 159,571, filed July 2, 1971,now U.S. Pat. No. 3,895,122.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to preservation of at least partially cookedpotato pieces by treatment with sulfur dioxide and packaging to excludemicrobial contamination and oxygen and to the related process.

2. Description of the Prior Art

Sulfur dioxide gas and sulfur dioxide-containing salts have beenincorporated in processes for the preservation of fresh fruits,vegetables, meats and the like, having low and intermediate moisturecontents, see U.S. Pat. No. 1,061,460, issued May 13, 1913. In thedehydration of most vegetables, sulfur dioxide is used, (usually in theform of solutions of a sodium salt) to prevent oxidation and enzymaticdiscoloration during processing and to protect the dried products fromnonenzymatic browning. In the drying of fruits, sulfur dioxide treatmentis known to bleach the dark pigments resulting from enzymatic browningand subsequently protect the dried fruit from nonenzymatic browning.

Sulfur dioxide heretofore has not been used commercially to microbiallystabilize partially cooked foods containing high moisture. The state ofthe art prior to this invention is summarized in the text Disinfection,Sterilization and Preservation Carl A. Lawrence and Seymour S. Block,Lea & Sebiger Publisher, 1968, which has an entire chapter devoted toGASEOUS STERILIZATION. The only reference to sulfur dioxide in thechapter is a statement at page 669 that it was used in the 19th Centuryas a means of fumigating sick rooms. Sulfur dioxide, however, isdiscussed at pages 639-640 in more detail in Chapter 39, entitled FOODPRESERVATIVES. There it is stated that sulfur dioxide is used to "treat"fruits and vegetables before dehydration. Low concentrations of sulfurdioxide are also used to delay the decay of fresh grapes and inhibitundesirable bacteria in wine making without interfering with yeastaction.

Other prior art teachings involve subjecting various fresh fruits andvegetables to sulfur dioxide under pressure in excess of 5 psig in aclosed chamber to enhance storage characteristics. Pre-peeled potatoeshave heretofore been treated with minor concentrations of sulfite saltsolutions to prolong their storage life under refrigeration. However,when significant concentrations of sulfur dioxide or its salts are usedon raw prepared potato pieces, substantial potato cell destructionoccurs, resulting in considerable fluid loss from the cells. As aconsequence, commercially available potato pieces are either canned,refrigerated, frozen or dehydrated.

The prior art does not disclose or suggest means for producing highmoisture potato products in the class of convenience foods which can bestored at ambient temperatures and which can be simply and quicklyutilized to make a variety of potato dishes.

Frozen french fried potatoes are by far the biggest volume frozen fooditem sold in the United States. Over 2 billion pounds were sold in 1959.Such commercially available parfried products must be kept frozenthroughout the distribution channels up to final heating prior toconsumption. The inherent high cost is accepted since no other means hasheretofore been developed for keeping the product microbially stablewithout such refrigeration. Prepeeled french fries are likewise sold intremendous quantities. Such products must be held under refrigerationand even then have a limited storage life. The present invention allowsambient distribution and storage temperatures prior to use.

SUMMARY OF THE INVENTION

It has now been found that sulfur dioxide-containing gelatinized (orcooked) potato pieces can be stored for prolonged periods without beingsubjected to extensive dehydration or refrigeration. More specifically,the present invention involves the discovery that uniform dispersion ofsulfur dioxide (SO₂) in a critically controlled concentration throughoutsuch potato pieces results in a product which can be packaged and storedat room temperature without microbial growth and without adverse effectson appearance and taste. Ultimate consumer use involves either nopreparation or merely heating and/or completion of cooking. The sulfurdioxide-containing potato pieces of this invention can be readily storedat ambient temperatures for periods in excess of 6 months.

It has been found that the present invention is useful even though thetreated potato pieces have a high residual moisture content resulting inwater activity far above about 0.85. This level of water activity hasbeen thought to be the maximum at which microbial stability can beattained by conventional methods of suppression of water activity, suchas is employed to preserve intermediate moisture food products, forexample, moist pet foods. The water activity of the gelatinized treatedpieces of this invention is 1.0. Thus the product is actually inequilibrium (at 100% relative humidity) with the surroundingenvironment.

Particularly advantageous results have been obtained where the ultimateconcentration of sulfur dioxide in the treated product is at least about200 parts per million (0.2%) with 500 to 700 parts per million beingespecially preferred. Although storage characteristics do not appear tobe adversely affected by larger concentrations of sulfur dioxide, it hasbeen found that where the ultimately dispersed sulfur dioxide level isgreater than 700 parts per million (0.07%), undesirable flavorcharacteristics develop when the potato pieces are subsequently reheatedby the consumer. The aforementioned concentrations of sulfur dioxide areintended to include both absorbed and free SO₂.

When the present invention is employed with an impervious container toexclude contaminating bacteria, it has been found that storage of thesulfur dioxide-containing potato pieces is further enhanced by packagingthem in an inert environment, i.e., either in a substantiallyoxygen-free gas such as nitrogen, carbon dioxide -- nitrogen mixtures,or in at least a partial vacuum. Special advantages have been obtainedwhere absorption of the sulfur dioxide by the potato pieces isaccomplished in a sealed container filled with a gaseous environmentcomposed essentially of sulfur dioxide and nitrogen. The package is alsopreferrably impermeable to moisture vapor and oxygen, thus maintainingboth the desired moisture level and absence of oxygen.

In still another aspect, it has been found that rate of absorption ofgaseous sulfur dioxide by the potato pieces is directly related to theirtemperature during treatment with sulfur dioxide. Thus the rate ofabsorption is sharply reduced when the potato pieces are subjected to atemperature reduction near freezing. As discussed in detail hereinafter,the resulting prolonged exposure of the outer surfaces of the individualpieces to gaseous sulfur dioxide environment maximizes inhibition ofbacterial and microbial growth on the surface thereof. The moreprolonged the contact with sulfur dioxide in the surrounding gaseousenvironment, the more complete is ultimate protection. This aspectappears to have the additional advantage of minimizing the concentrationof gaseous sulfur dioxide necessary to accomplish microbial stability.

Following the prolonged storage at ambient temperatures, the treatedpieces can be either used directly without further treatment or preparedfor consumption by refrying in deep fat, pan frying for a brief periodof time such as 3-5 minutes or heating in an oven at approximately 450°F. for up to about 15 minutes. Other methods of finishing the potatopieces of this invention subsequent to storage will be apparent to oneskilled in this art.

Although it is not intended that the present invention be limited to anytheoretical concept, it appears that substantially fully gelatinizedpotato pieces interact differently with sulfur dioxide (SO₂) than doeither fresh potato pieces or those that have merely been waterblanched. Raw potato pieces exposed to SO₂ gas have been found toimmediately lose significant amounts of water, apparently as a result ofthe SO₂ and free water (in the potato cells) forming sulfurous acidwhich in turn destroys further cell walls. Such treated pieces areunacceptable in flavor and texture. Even where raw potato pieces arefirst blanched (partially gelatinized) in water at elevated temperaturesand then exposed to SO₂, some water losses have been noted along withthe formation of the aforementioned off flavor and leathery texture.Surprisingly, fully gelatinized potato pieces that have been exposed toat least the aforementioned critical minimum levels of SO₂ do notexhibit any discernible water loss, nor is microbial growth present evenafter extended periods of storage at room temperature.

In heating steps such as gelatinization (cooking), starch gelatinizationties up the natural water content of the potato. Sulfur dioxide israpidly absorbed and tightly held by the gelatinized starch in contrastto the loose binding between sulfur dioxide and raw starch as shown bythe fact that sulfur dioxide treated raw potato pieces can be purged ofmuch of the sulfur dioxide content by vacuum treatment, whereas, thesulfur dioxide content of substantially fully gelatinized pieces is onlyslightly reduced by a vacuum treatment. As a result, SO₂ is highlysatisfactory for the room temperature stabilization of gelatinized orcooked potato pieces while unsatisfactory for fresh or pre-blanchedpotato pieces.

The term "potato piece" is intended to include whole peeled or unpeeledpotatoes, diced pieces, strips, slices, potato hollows and the like. Theterms "gelatinized" or cooked are intended to mean potato pieces thathave been subjected to sufficiently prolonged high temperatures (inwater or oil) to produce substantially complete gelatinization. When thepotato pieces are subjected to at least partial cooking such as inconventional-type vegetable oils at temperatures of at least about 200°F for at least one minute, they are described as being "parfried". Itwill be obvious to one skilled in this art that various combinations ofcooking temperatures and time can be utilized. Temperatures have beenvaried from 200°-350° F and cooking time from 1-12 minutes with frenchfry-type strips. Longer frying time tends to decrease the moisturecontent of the pieces and increases oil content while shorter fryingtime has the converse effect. The term "blanching" is intended to meanincomplete gelatinization by treating in water at about 180° F for brieftime periods.

The novel product of this invention can be formed by treatment withsulfur dioxide both in the gaseous form and as a sulfurous acidsolution. In order to insure that the interior of parfried potato pieceswill be completely protected from the growth of microorganisms, it isnecessary not only to employ a sufficient amount of sulfur dioxide, butto allow it to be absorbed generally uniformly throughout the piece bycontacting all surfaces of the potato piece with SO₂. When a solution isemployed, the sulfur dioxide absorption is accomplished by dipping thegelatinized or cooked pieces into the solution for a brief time such asa few minutes.

Generally speaking, when sulfur dioxide is introduced as a gas, theprocess can involve either the addition of sulfur dioxide to theultimate package or the pieces can be tumbled in an SO₂ environment(such as would be provided in an air-tight rotatable drum) for a timesufficient to enable the pieces to absorb the prerequisite amount ofsulfur dioxide gas after which the pieces are transferred to a packagewhich is swept free of oxygen by flushing with inert gas before sealing.Absorption of the quantities of sulfur dioxide set forth herein toproduce prolonged product microbial stability at ambient temperaturedecreases the pH of the potato pieces from about 5.7-6.3 to about5.1-5.3.

It has been found that in the parfrying embodiment of this invention,potatoes with sugars as high as 3% can be utilized without any sugarreduction treatment. In prior art, frozen parfried products with sugarlevels of 2.0% or lower were required and these levels were attained byconditioning raw potatoes by high temperature storage or by leachingsugars from the pieces in water as by a blanching step. By including asimilar blanching step in the process of this invention, potatoes withsugar contents as high as 5% can be preserved successfully by reducingtheir sugar level only to about 3.0%.

When the potato pieces are prepared by parfrying, the amount of oilretained by the parfried pieces can be varied widely, although generallythe conventional level is from 5 to 15% by total weight. It will beapparent to one skilled in this art that the deep fat frying can beaccomplished with any of the commercially available animal fats andvegetable oils that are used in such cooking procedures.

For the reason that sulfur dioxide has a bleaching effect on the deepfat fried potato pieces, it is most desirable to provide additionalcoloring for the pieces following parfrying through the application of aheated coloring oil such as Vegetone, a natural color extract. The mostdesirable coloring has been attained when a coloring pigment such asVegetone Popcorn Color, available from Kalamazoo Spice ExtractionCompany, in an amount of 0.025-0.050% by weight was suspended in Taskoil (a trade name of Hunt-Wesson for an animal-vegetable fat) and theoil suspension heated to approximately 100°-250° F.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples are intended to be representative of the processand product of this invention. It will be apparent that the details ofthe examples are not to be regarded as limitations, as they may bevaried, as will be understood by one skilled in this art. All testingwas performed at room temperature (about 75° F) and a barometricpressure of about 672 mm mercury. To determine concentrations of SO₂ atstandard conditions, the following volume amounts would have to bemodified by a factor of about 0.813.

EXAMPLE IA

Fresh potatoes were washed, peeled, trimmed and cut into french frystrips approximately three-eighths inch by three-eighths inch incross-section. The odd sized pieces were removed and the remainingproperly sized strips washed to remove any free starch. The strips wereblanched in 180° F water for five minutes and drained. The strips werethen parfried by submerging in Task oil at 320° F for 4 minutes. Theparfried pieces were dipped in 200° F Task oil containing 0.05%Vegetone, drained and cooled near freezing after which one poundportions were added to impermeable flexible bags. Air was removed fromthe bags by displacement (sweeping) with nitrogen gas after which thebags were heat sealed. 0.290 g. (1 Milliliters) of sulfur dioxide wasinjected into each bag which was then resealed.

The bags were stored at room temperature and examined periodically overa period of six months for microbial growth, appearance and aroma.Measurable sulfur dioxide concentration was found to be about 500 partsper million at time of packaging. The products were found to be stablein all instances. At periodic intervals, strips were removed from thebags and were oven heated at 450° F for 15 minutes. Large consumerpanels judged them to be comparable to those made directly from freshpotatoes and markedly superior to those made from other commerciallyavailable potato products.

EXAMPLE IB

Applying a process similar to that of Example IA, the parfried potatoeswere drained and cooled. No coloring dip was employed. After treatmentand storage, following the procedure of Example IA, at periodicintervals, bags of the strips were refried in deep fat. As in theprevious case, consumer panels judged the finished product excellent inquality.

EXAMPLE IC

Applying the process set forth in Example IA, the prepared strips weresubjected to a 3-minute water blanch at 180° F and then air dried to50-65% moisture content. The blanched predried pieces were thenpackaged, gas treated and stored as in Example IB. No parfrying wasemployed. Upon subsequent panel testing, the product was ratedsatisfactory. (When the air drying step was eliminated from the aboveprocedure, the finished product produced was of leathery texture.)

EXAMPLE ID

Potato strips were prepared and parfried as set forth in Example IB. Theparfried pieces were then gently tumbled in an atmosphere of sulfurdioxide gas until the SO₂ content of the strips was in the range ofabout 500 parts per million. The treated strips were introduced in onepound portions into the aforementioned gas-impermeable, heat-sealablebags and the air removed therefrom by sweeping the bags with nitrogen.The bags were then sealed and stored at room temperatures, i.e.,approximately 65° F. As in the previous examples, the parfried productwas found to be microbially stable after prolonged storage periods andexhibited excellent quantity when refried to the finished french friedproduct.

EXAMPLE IE

Potato strips parfried in accordance with Example IB were subjected todipping in an aqueous solution containing approximately 2% sulfurousacid, for about 2 minutes. The strips were drained and packaged in aninert environment of nitrogen. After various lengths of storage, sampleswere finish fried and found to have an acceptable flavor.

EXAMPLE IIA

Whole, unpeeled potatoes were washed and scrubbed to remove dirt andloose corky material; then cooked in Task oil at 250° F for 40 minutes.The cooked potatoes were then cooled at room temperature, packaged inthe impermeable flexible bags and flushed with N₂ gas and sealed. 105 mlSO₂ per pound of cooked potatoes was injected into the bags which werethen resealed and stored at room temperature. 105 ml equals 0.244 grams.

EXAMPLE IIB

The procedure of Example IIA was repeated except the scrubbed potatoeswere cooked by baking in hot air in an oven. They were then cooled,packaged and gas treated as before.

EXAMPLE IIC

Small peeled potatoes were cooked in oil at 250° F for 30 minutes. Thecooked potatoes were cooled, packaged and handled in the same way asdescribed in Example IIA.

EXAMPLE IID

The procedure of Example IIA was followed except the potatoes werecooked in atmospheric steam for 50 minutes.

EXAMPLE IIE

The procedure of Example IID was duplicated except small peeled potatoeswere cooked in atmospheric steam for 35 minutes.

EXAMPLE IIF

Potatoes were peeled; diced into 1/2 inch cubes; washed with water, andthen blanched in water at 180° F for 4 minutes. The blanched cubes werethen fried in oil for 4 minutes at 250° F; cooled to room temperature;and packaged under N₂ in the impermeable flexible bags and sealed. 105ml SO₂ per pound of parfried cubes was injected into the pack and sealedand stored as in previous samples. 105 ml equals 0.244 grams.

After prolonged storage, the products of Examples IIA through IIF werefound to be microbially stable and of excellent quality. The product ofExample IIF is ideally suited for use in potato salad for example, withno required treatment. The product of Example IIA needs only be heatedfor a baked potato. Other uses for these new convenience potato productswill be obvious to one skilled in this art.

Although the above examples disclose processes which produce microbiallystable products of excellent quality even after prolonged ambienttemperature storage, exhaustive studies were conducted to determine theparameters affecting both microbial stability and quality.

The concentration of sulfur dioxide was found to be critical. When thetreated products contained more than 700 parts per million SO₂, theflavor was objectionable. When 0.203 g. or less sulfur dioxide gas wasadded to one pound of cooled parfried french fry strips, mold growthoften developed after about two weeks of storage at room temperature.When 0.244 g. or more sulfur dioxide was added to and absorbed by onepound of cooled parfried strips, stability was attained in all cases.However, when the cooled strips were inoculated with Aspergillus,Rhizopus, and Penicillium mold cells at the level of about 1000 cellsper gram product, some mold development was encountered in the storedproduct. It was found that the use of 0.406 g.SO₂ per pound of productresulted in microbial stability even in inoculated packs. Similarinoculation was done using strains of Bacillus cereus, Staphylococcusaureus, Bacillus polymyxa, Clostridium perfringens, Salmonellatyphymurium, Erivinia carotovora, and coliform bacteria. In all cases,where bacterial inoculation was employed, stability was attained when aslittle as 0.290 g.SO₂ per pound of product was added.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity andunderstanding, it will be apparent to one skilled in the art thatcertain changes and modifications may be practiced within the spirit ofthe invention.

What is claimed is:
 1. A novel article of manufacture,comprising:substantially fully gelatinized potato pieces having a wateractivity of above about 0.85 containing about 200 to 700 parts permillion of absorbed sulfur dioxide gas; a sealed container enclosingsaid pieces, said container being substantially impermeable to microbesand oxygen; and a substantially oxygen-free environment occupying thespace in said container surrounding said pieces.
 2. An article ofmanufacture in accordance with claim 1 wherein the amount of absorbedsulfur dioxide is from about 500-700 parts per million.
 3. The articleof manufacture of claim 1 in which said substantially fully gelatinizedpotato pieces are parfried.
 4. The novel article of manufacture inaccordance with claim 1 in which said oxygen-free environment comprisesa gas selected from the group consisting of nitrogen, carbon dioxide andmixtures thereof.
 5. The novel article of manufacture in accordance withclaim 1 in which said oxygen-free environment comprises at least apartial vacuum.
 6. The novel article of manufacture in accordance withclaim 1 in which said gelatinized potato pieces are selected from thegroup consisting of whole unpeeled potatoes, whole peeled potatoes, andpotato pieces.